Hydrotreatment catalysts generally comprising an amorphous or crystalline oxide support such as an alumina, a silica, a silica-alumina or a zeolite on which at least one element from groups VIII and VI of the periodic table or a combination of several elements from these same groups is deposited, such as solids designated CoMo/Al2O3, NiMo/Al2O3 or NiW/Al2O3, must first be sulphurised to endow them with catalytic performances for all hydrotreatment reactions such as hydrodesulphurisation, hydrodenitrogenation, demetallisation and certain hydrogenation reactions. This sulphurisation step prior to catalysis proper can be carried out in different manners.
The first manner, known as in situ sulphurisation, is characterized by the fact that the catalyst in its oxide form is charged into the hydrocarbon conversion reactor first for sulphurisation therein. The second manner, known as off-site pre-sulphurisation, as described in a variety of the Applicant""s patents (U.S. Pat. Nos. 4,719,195, 5,397,756, European patent EP-A-0 785 022), differ from the preceding manner in that sulphurisation or pre-sulphurisation of the catalyst is carried out in a particular unit which is distinct from the hydrotreatment reactor.
European patent application EP-A1-0 745 660 describes a coking step carried out after the catalyst pre-sulphurisation phase, which are both carried out in situ, i.e., inside the hydrocarbon conversion reactor. That process for post-coking catalysts for hydrotreating gasoline cuts from catalytic cracking containing sulphur-containing compounds and olefinic compounds describes selective poisoning of active hydrogenating sites which, while maintaining the hydrodesulphurising properties of the catalyst, limit hydrogenation of the olefins responsible for the octane number of such gasolines.
The aim of the present invention is to carry out pre-sulphurisation of the catalyst in the presence of hydrogen and at least one sulphur-containing compound which may be hydrogen sulphide or any other sulphur-containing compound which can generate hydrogen sulphide by hydrogenolysis, characterized in that in order to improve the catalytic performances in the hydrotreatment reactions, the catalyst is pre-carbonised so as to deposit solid carbon in the pores of the catalyst, the major portion of said carbon being non leachable. The process of the invention is of particular application to pre-sulphurisation carried out off-site.
The present invention, characterized by carrying out pre-carbonisation, can improve the hydrodesulphurising and hydrogenating properties of the catalyst. Further, it has been shown that this pre-carbonisation can reduce the initial selectivity for the cracking product and for isomerisation. One explanation may be that this drop in selectivity for cracking and for isomerisation is due to the carbon deposit attenuating the acidity of the support. Catalyst deactivation can thus be reduced and its service life increases.
Pre-carbonisation can be carried out prior to the pre-sulphurisation phase or simultaneously with said pre-carbonisation step, or even before and simultaneously with said pre-sulphurisation step. When pre-carbonisation is carried out prior to pre-sulphurisation of the catalyst off-site, then any technique which can deposit solid carbon in a mainly non leachable form and homogeneously in the pores of the catalyst may be suitable, such as dry impregnation or chemisorption, both combined with heat treatment.
For impregnation carried out at ambient temperature, the liquid carbon-containing compound is introduced into the pores of the catalyst without necessarily filling the pore volume of the catalyst completely. Thus the impregnation volume is partially filled to between 10% and 100%, preferably in the range 20% to 90%. Suitable carbon-containing sources can be selected from paraffinic, naphthenic and/or aliphatic hydrocarbons, and also from organic oxygen-containing compounds such as alcohols, ketones, aldehydes, organic acids, fatty acids and vegetable oils. These compounds are characterized by high boiling points, for example between 150xc2x0 C. and 500xc2x0 C., preferably over 200xc2x0 C. The quantity of carbon deposited at the end of the impregnation step is in the range 2% to 30% by weight with respect to the mass of oxide catalyst.
Following the impregnation step, a heat treatment step is necessary, this heat treatment step being carried out in nitrogen or another inert gas, at a temperature in the range 150xc2x0 C. to 650xc2x0 C., preferably in the range 300xc2x0 C. to 500xc2x0 C. or in air at a temperature in the range 50xc2x0 C. to 400xc2x0 C., preferably at a temperature of less than 350xc2x0 C. The aim of this subsequent heat treatment, or pyrolysis, preferably carried out in an inert gas, is twofold. Firstly it has the function of re-adjusting the amount of carbon to between 2% and 10% by weight, preferably in the range 3% to 7% by weight with respect to the oxide matrix, and secondly to transform the hydrocarbons, which may or may not contain oxygen, introduced during the impregnation step, such that the residual carbon, in an amount in the range 2% by weight to 10% by weight and preferably in the range 3% by weight to 7% by weight with respect to the oxide, is a residual carbon-containing compound the major portion of which is not leachable in refluxed toluene. The major portion of the residual carbon-containing compound is not leachable in refluxed toluene, i.e., the degree of leaching is less than 40%, preferably less than 30% and more preferably less than 20%.
Pre-carbonisation can also be carried out by direct reaction between the solid and a hydrocarbon compound. It is possible to use a compound which is quick to polymerise, such as olefins or diolefins at a temperature in the range 20xc2x0 C. to 400xc2x0 C.
Carbonisation can also be carried out simultaneously with the catalyst sulphurisation step, the gaseous carbon-containing compound being introduced into the gas phase simultaneously with the sulpho-reducing mixture constituted by hydrogen and hydrogen sulphide. Suitable carbon-containing compounds for this pre-carbonisation method are compounds which are quick to polymerise such as olefins or diolefins.
For this catalyst pre-carbonisation method which is simultaneous with pre-sulphurisation, the carbonisation gas can be introduced into the gas phase via an injection point which is either identical to or different from that used to introduce the sulphurising mixture. In the latter case, and for example when using a rotating cylindrical furnace, this displacement of the carbonisation mixture introduction point with respect to the sulpho-reducing mixture introduction point can be radial or axial. It is preferably carried out such that the carbonisation phase is carried out in a temperature range which is lower than the maximum sulphurisation temperature.
The pre-carbonised catalyst sulphurisation step can be carried out at atmospheric pressure in a reactor heated to between 200xc2x0 C. and 600xc2x0 C., preferably in the range 200xc2x0 C. to 500xc2x0 C., in the presence of a sulpho-reducing gas mixture. More particularly, the method of the invention is applicable to off-site sulphurisations which can be carried out at high temperature, for example at temperatures of more than 400xc2x0 C. The reactor used can be a fixed bed type reactor or a system using moving bed technology such as a rotating bed, a fluidised bed or a sinking bed. The sulpho-reducing gaseous mixture is constituted by hydrogen and hydrogen sulphide, with a hydrogen sulphide partial pressure in the range 5000 to 71000 Pascals. The hydrogen sulphide is either generated in the reaction chamber by hydrogenolysis of a sulphur-containing compound in the presence of hydrogen (nascent hydrogen sulphide), such as thiols, organic sulphides, organic disulphides and organic polysulphides, or introduced simultaneously with the hydrogen. In the case where moving bed technology is used, pre-sulphurisation is said to be co-current if the sulpho-reducing mixture is introduced at the catalyst injection point, and it is said to be counter-current if the gaseous mixture is introduced at the catalyst exit point.